In mobile radiocommunication systems, the capability is provided to transfer the handling of communications with a mobile station from one base station to another, as the mobile station changes its position and so moves out of the coverage range of one base station and into the coverage area of another base station. This process is commonly termed handover or handoff.
To smoothly complete a handover, the network controlling the base stations must first determine, for each mobile station, whether the need for handover is imminent and secondly determine to which new base station handover should be effected. In making the latter decision it is desirable that the network controller know either how well each base station can receive signals from a mobile station in question, or how well the mobile station in question can receive signals from each base station, or both.
Conventional mobile telephone systems were based largely on Frequency Division Multiple Access (FDMA), in which each base station transmits on a unique frequency within its current base station area. The mobile station is thus unaware of signals on other frequencies from surrounding bases. In FDMA systems it would be too costly to equip mobile stations with an extra receiver that could be used to scan other base frequencies. Instead, it was established practice that base stations are equipped with a scanning receiver that looks out for the signals of approaching mobile stations. The network then hands over a mobile from a base station covering an area it is leaving to the base station that reports the best reception of the mobile station's signal.
More recent cellular telephone standards employ Time Division Multiple Access (TDMA) in which a fixed time period (e.g., 20 mS) on each radio frequency is divided into a number (e.g., 3) of short timeslots (e.g., 6.6 mS) that are cyclically used by different mobile stations. Thus, a first mobile station transmits in the first timeslot in each period, a second mobile station transmits in the second timeslot in each period and so on. Likewise the base station transmits to one mobile station in the first timeslot, another mobile station in the second slot and so on. By offsetting the allocation of timeslots in the two communications directions, base to mobile (the downlink) and mobile to base (the uplink), it can be arranged that a first mobile transmits in the first timeslot and receives in the second timeslot; a second mobile transmits in the second timeslot and receives in the third, while a third mobile transmits in the third timeslot and receives in the first timeslot. An advantage of this arrangement is that a mobile station does not need to transmit and receive simultaneously, which facilitates sharing a single antenna.
In the above three-timeslot example, each mobile station is active to transmit or receive in two of the three timeslots and idle in the remaining timeslot. Therefore it is possible for TDMA mobile stations to use this idle time to search for signals from other base stations and measure their signal strength. By reporting these signal strength measurements to the base station using a low speed data channel multiplexed with the traffic (i.e., voice), the network is informed about the base stations each mobile station can receive. The network can use this information to effect handover to the best base station, and such a system is termed mobile assisted handover (MAHO). When the base stations scan for the signal strength of mobile stations, the system could be termed base assisted handover (BAHO). Several types of handoff or handover can be executed by a mobile station under control of the intelligent network controller.
For example, a first type, called an internal handover, occurs when the network decides to keep the mobile station connected to its current base station for traffic sharing or capacity optimization reasons, but to switch transmissions from the mobile station to another frequency or timeslot. If a frequency or timeslot change is commanded, a slight interruption in traffic will occur while the mobile station acquires synchronization on the new frequency.
A second type of handover, called external handover, occurs when the network decides to transfer the mobile station to another base station, which in conventional mobile telephone systems implies change of frequency or timeslot. This may also introduce a slight interruption in traffic and a risk of losing the call if the new radio channel experiences interference.
A third type of handover is the transfer of a mobile station to another base station without a frequency change. This handover has the potential to be a so-called glitch-free or seamless handover that involves no interruption of traffic whatsoever. Systems in which multiple base stations are simultaneously connected to one mobile station to provide information relating to the same connection are commonly called macrodiversity or soft handover systems. The base stations transmit the same signal to the mobile station which then combines the two signals.
One way in which conventional macrodiversity systems operate is to use the same communication channel between a mobile station and all base stations involved in a particular connection. A communication channel can be characterized by at least one of its frequency, timeslot or code depending on the multiple access method employed by the system, e.g., FDMA, TDMA and CDMA. In these types of simulcast systems, signals from the various base stations are handled by the mobile as different transmission paths.
Another method in which macrodiversity systems operate is to use different communication channels between the mobile station and the various base stations. In TDMA systems, this means different timeslots and/or frequencies. Unfortunately, several existing TDMA radiocommunication systems, e.g., GSM and ADC, have relatively few timeslots per frame. Typically these timeslots are already fully used by the mobile station for reception of signals from a base station, transmission of signals to the base station and MAHO measurements of signal quality of adjacent base stations.
Since there is no capacity left for handling another communication channel on another timeslot, achieving macrodiversity in such TDMA systems generally requires using the same timeslot on the same or a different frequency as another communication channel. An approach which uses the same timeslot on the same frequency has the drawback that it uses synchronized base stations.
Using the same timeslot on a different frequency implies that the mobile station can only receive one of the base stations at a time. Thus, the mobile selects one of the base stations based on, for example, the quality of the last received timeslot.